Tiltrotor aircraft having spherical bearing mounted pylon assemblies

ABSTRACT

A propulsion system for a tiltrotor aircraft includes an engine supported by the airframe and a fixed gearbox operably coupled to the engine. Inboard and outboard pedestals are supported by the airframe and positioned above the wing. A pylon assembly is rotatably coupled between the inboard and outboard pedestals. The pylon assembly includes a spindle gearbox having an input gear, a mast operably coupled to the input gear and a proprotor assembly operable to rotate with the mast. The spindle gearbox is rotatable about a conversion axis to selectively operate the tiltrotor aircraft between helicopter and airplane modes. A common shaft, rotatable about the conversion axis, is configured to transfer torque from an output gear of the fixed gearbox to an input gear of the spindle gearbox. Each of the inboard and outboard pedestals includes a spherical bearing providing a self-aligning coupling with the pylon assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of co-pending application Ser. No.13/966,726 filed Aug. 14, 2013.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to tiltrotor aircraftoperable for vertical takeoff and landing in a helicopter mode andforward cruising in an airplane mode and, in particular, to tiltrotoraircraft having a fixed engine and rotatable pylon assemblyimplementation.

BACKGROUND

Fixed-wing aircraft, such as airplanes, are capable of flight usingwings that generate lift responsive to the forward airspeed of theaircraft, which is generated by thrust from one or more jet engines orpropellers. The wings generally have an airfoil cross section thatdeflects air downward as the aircraft moves forward, generating the liftforce to support the aircraft in flight. Fixed-wing aircraft, however,typically require a runway that is hundreds or thousands of feet longfor takeoff and landing.

Unlike fixed-wing aircraft, vertical takeoff and landing (VTOL) aircraftdo not require runways. Instead, VTOL aircraft are capable of takingoff, hovering and landing vertically. One example of a VTOL aircraft isa helicopter which is a rotorcraft having one or more rotors thatprovide lift and thrust to the aircraft. The rotors not only enablehovering and vertical takeoff and landing, but also enable forward,backward and lateral flight. These attributes make helicopters highlyversatile for use in congested, isolated or remote areas. Helicopters,however, typically lack the forward airspeed of fixed-wing aircraft dueto the phenomena of retreating blade stall and advancing bladecompression.

Tiltrotor aircraft attempt to overcome this drawback by utilizingproprotors that can change their plane of rotation based on theoperation being performed. Tiltrotor aircraft typically have a pair ofnacelles mounted near the outboard ends of a fixed wing with eachnacelle housing a propulsion system that provides torque and rotationalenergy to a proprotor. The nacelles are rotatable relative to the fixedwing such that the proprotors have a generally horizontal plane ofrotation providing vertical thrust for takeoff, hovering and landing,much like a conventional helicopter, and a generally vertical plane ofrotation providing forward thrust for cruising in forward flight withthe fixed wing providing lift, much like a conventional propeller drivenairplane. It have been found, however, that the outboard location of thenacelles coupled with the requirement of rotating the nacellessignificantly influence the size and weight of the airframe structurerequired to support the nacelles. Accordingly, a need has arisen forimproved systems and methods for realizing a tiltrotor aircraft havingreduced structural loads generated by the propulsion system.

SUMMARY

In a first aspect, the present disclosure is directed to a propulsionsystem for a tiltrotor aircraft including an engine supported by theairframe proximate an outboard end of the wing and a fixed gearboxoperably coupled to the engine and having an output gear. Inboard andoutboard pedestals are supported by the airframe above the wing. A pylonassembly is rotatably coupled between the inboard and outboardpedestals. The pylon assembly includes a spindle gearbox having an inputgear, a mast operably coupled to the input gear and a proprotor assemblyoperable to rotate with the mast. The spindle gearbox is rotatable abouta conversion axis to selectively operate the tiltrotor aircraft betweenhelicopter and airplane modes. A common shaft is configured to transfertorque from the output gear of the fixed gearbox to the input gear ofthe spindle gearbox. The common shaft is rotatable about the conversionaxis. Each of the inboard and outboard pedestals includes a sphericalbearing providing a self-aligning coupling between the pylon assemblyand the inboard and outboard pedestals to reduce alignment sensitivitybetween the inboard and outboard pedestals.

In some embodiments, the coupling between the pylon assembly and theoutboard pedestal may be a fixed bearing coupling to substantiallyprevent lateral movement of the pylon assembly relative to the outboardpedestal. In certain embodiments, the coupling between the pylonassembly and the inboard pedestal may be a floating bearing coupling toallow lateral movement of the pylon assembly relative to the inboardpedestal. In some embodiments, the pylon assembly may include an inboardsleeve positioned within the spherical bearing of the inboard pedestaland an outboard sleeve positioned within the journal spherical of theoutboard pedestal. In certain embodiments, the spherical bearings mayenable the spindle gearbox to rotate about the conversion axis in anenvironment including misalignment of the inboard and outboardpedestals.

In certain embodiments, the inboard and outboard pedestals may be fullpillow block housings. In other embodiments, the inboard and outboardpedestals may be split pillow block housings. In additional embodiments,the inboard pedestal may be a full pillow block housing while theoutboard pedestal may be a split pillow block housing. In furtherembodiments, the inboard and outboard pedestals may be tip ribsextending above the wing and defining slots having bearing cartridgesincluding bearing assemblies received therein. In some embodiments, theinboard and outboard pedestals support fore/aft loads generated by theproprotor assembly when the tiltrotor aircraft is in the airplane modeand vertical loads generated by the proprotor assembly when thetiltrotor aircraft is in the helicopter mode. In certain embodiments,each of the spherical bearings may include a monoball bearing and aspherical race. In some embodiments, the fixed gearbox may be coupled tothe outboard pedestal to maintain the output gear of the fixed gearboxin substantial collinear alignment with the input gear of the spindlegearbox.

In a second aspect, the present disclosure is directed to a tiltrotoraircraft having a helicopter mode and an airplane mode. Tiltrotoraircraft includes an airframe including a fuselage and a wing. An engineis supported by the airframe proximate an outboard end of the wing. Afixed gearbox is operably coupled to the engine and has an output gear.Inboard and outboard pedestals are supported by the airframe above thewing. A pylon assembly is rotatably coupled between the inboard andoutboard pedestals. The pylon assembly includes a spindle gearbox havingan input gear, a mast operably coupled to the input gear and a proprotorassembly operable to rotate with the mast. The spindle gearbox isrotatable about a conversion axis to selectively operate the tiltrotoraircraft between helicopter and airplane modes. A common shaft isconfigured to transfer torque from the output gear of the fixed gearboxto the input gear of the spindle gearbox. The common shaft is rotatableabout the conversion axis. Each of the inboard and outboard pedestalsincludes a spherical bearing providing a self-aligning coupling betweenthe pylon assembly and the inboard and outboard pedestals to reducealignment sensitivity between the inboard and outboard pedestals.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIG. 1 is a perspective view of a tiltrotor aircraft in airplane mode inaccordance with embodiments of the present disclosure;

FIG. 2 is a perspective view of a tiltrotor aircraft in helicopter modein accordance with embodiments of the present disclosure;

FIG. 3 is a perspective view of a tiltrotor aircraft in airplane mode inaccordance with embodiments of the present disclosure;

FIG. 4 is a perspective view of a propulsion system of a tiltrotoraircraft in accordance with embodiments of the present disclosure;

FIG. 5 is a cross sectional view of a pylon assembly of a tiltrotoraircraft in accordance with embodiments of the present disclosure;

FIG. 6 is an aft view of a propulsion system and wing section of atiltrotor aircraft in accordance with embodiments of the presentdisclosure;

FIG. 7 is a top view of a propulsion system and wing section of atiltrotor aircraft in accordance with embodiments of the presentdisclosure;

FIG. 8 is a perspective view of a wing section of a tiltrotor aircraftin accordance with embodiments of the present disclosure;

FIG. 9 is a perspective view of a propulsion system of a tiltrotoraircraft in accordance with embodiments of the present disclosure;

FIG. 10 is a cross sectional view of a propulsion system section of atiltrotor aircraft in accordance with embodiments of the presentdisclosure;

FIG. 11 is a cross sectional view of a propulsion system section of atiltrotor aircraft in accordance with embodiments of the presentdisclosure;

FIG. 12 is a perspective view of a quill shaft in accordance withembodiments of the present disclosure;

FIG. 13 is a perspective view of a propulsion system section of atiltrotor aircraft in a partially disassembled state in accordance withembodiments of the present disclosure;

FIG. 14 is a perspective view of a propulsion system section of atiltrotor aircraft in a partially disassembled state in accordance withembodiments of the present disclosure;

FIGS. 15A-15B are perspective and exploded views of a pylon assemblypositioned above a wing between inboard and outboard pedestals inaccordance with embodiments of the present disclosure;

FIGS. 16A-16B are perspective and exploded views of a pylon assemblypositioned above a wing between inboard and outboard pedestals inaccordance with embodiments of the present disclosure;

FIGS. 17A-17B are perspective and exploded views of a pylon assemblypositioned above a wing between inboard and outboard pedestals inaccordance with embodiments of the present disclosure;

FIGS. 18A-18B are perspective and exploded views of a pylon assemblypositioned above a wing between inboard and outboard pedestals inaccordance with embodiments of the present disclosure;

FIG. 19 is a cross sectional view of a pylon assembly positioned above awing between inboard and outboard pedestals in accordance withembodiments of the present disclosure; and

FIG. 20 is a cross sectional view of a pylon assembly positioned above awing between inboard and outboard pedestals in accordance withembodiments of the present disclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,not all features of an actual implementation may be described in thepresent disclosure. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would be a routine undertakingfor those of ordinary skill in the art having the benefit of thisdisclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicedescribed herein may be oriented in any desired direction. In addition,as used herein, the term “coupled” may include direct or indirectcoupling by any means, including moving and/or non-moving mechanicalconnections and the term “pedestal” will refer to the structure abovethe wing to which the pylon assembly is mounted.

Referring to FIGS. 1-3 in the drawings, a tiltrotor aircraft isschematically illustrated and generally designated 10. Aircraft 10includes a fuselage 12, a wing mount assembly 14 that is rotatablerelative to fuselage 12 and a tail assembly 16 including rotatablymounted tail members 16 a, 16 b having control surfaces operable forhorizontal and/or vertical stabilization during forward flight. A wing18 is supported by wing mount assembly 14 and rotates with wing mountassembly 14 relative to fuselage 12 to enable tiltrotor aircraft 10convert to a storage configuration. Together, fuselage 12, tail assembly16 and wing 18 as well as their various frames, longerons, stringers,bulkheads, spars, ribs, skins and the like may be considered to be theairframe of tiltrotor aircraft 10.

Located proximate the outboard ends of wing 18 are propulsion assemblies20 a, 20 b. Propulsion assembly 20 a includes a fixed nacelle 22 a thathouses an engine and a fixed portion of the drive system. In addition,propulsion assembly 20 a includes a pylon assembly 24 a that ispositioned inboard of fixed nacelle 22 a and above wing 18. Pylonassembly 24 a is rotatable relative to fixed nacelle 22 a and wing 18between a generally horizontal orientation, as best seen in FIG. 1, agenerally vertical orientation, as best seen in FIG. 2. Pylon assembly24 a includes a rotatable portion of the drive system and a proprotorassembly 26 a that is rotatable responsive to torque and rotationalenergy provided via the engine and drive system. Likewise, propulsionassembly 20 b includes a fixed nacelle 22 b that houses an engine and afixed portion of the drive system. In addition, propulsion assembly 20 bincludes a pylon assembly 24 b that is positioned inboard of fixednacelle 22 b and above wing 18. Pylon assembly 24 b is rotatablerelative to fixed nacelle 22 b and wing 18 between a generallyhorizontal orientation, as best seen in FIG. 1, a generally verticalorientation, as best seen in FIG. 2. Pylon assembly 24 b includes arotatable portion of the drive system and a proprotor assembly 26 b thatis rotatable responsive to torque and rotational energy provided via theengine and drive system.

FIGS. 1 and 3 illustrate aircraft 10 in airplane or forward flight mode,in which proprotor assemblies 26 a, 26 b are rotating in a substantiallyvertical plane to provide a forward thrust enabling wing 18 to provide alifting force responsive to forward airspeed, such that aircraft 10flies much like a conventional propeller driven aircraft. FIG. 2illustrates aircraft 10 in helicopter or VTOL flight mode, in whichproprotor assemblies 26 a, 26 b are rotating in a substantiallyhorizontal plane to provide a lifting thrust, such that aircraft 10flies much like a conventional helicopter. It should be appreciated thataircraft 10 can be operated such that proprotor assemblies 26 a, 26 bare selectively positioned between airplane mode and helicopter mode,which can be referred to as a conversion flight mode. Even thoughaircraft 10 has been described as having one engine in each fixednacelle 22 a, 22 b, it should be understood by those having ordinaryskill in the art that other propulsion system arrangements are possibleand are considered to be within the scope of the present disclosureincluding, for example, having a single engine which may be housedwithin one of the fixed nacelles or within the fuselage that providestorque and rotational energy to both proprotor assemblies 26 a, 26 b.

During all flight modes, proprotor assemblies 26 a, 26 b rotate inopposite directions to provide torque balancing to aircraft 10. Forexample, when viewed from the front of aircraft 10 in forward flightmode, proprotor assembly 26 a rotates clockwise and proprotor assembly26 b rotates counterclockwise. In the illustrated embodiment, proprotorassemblies 26 a, 26 b each include three twisted proprotor blades thatare equally spaced apart circumferentially at approximately 120 degreeintervals. It should be understood by those having ordinary skill in theart, however, that the proprotor assemblies of the present disclosurecould have proprotor blades with other designs and other configurationsincluding proprotor assemblies having four, five or more proprotorblades. Further, it should be understood by those having ordinary skillin the art that even though propulsion systems 20 a, 20 b areillustrated in the context of tiltrotor aircraft 10, the propulsionsystems of the present disclosure can be implemented on other types oftiltrotor aircraft including, for example, quad tiltrotor aircraft andunmanned tiltrotor aircraft, to name a few.

Referring now to FIGS. 4-11, propulsion assembly 20 a is disclosed infurther detail. Propulsion assembly 20 a is substantially similar topropulsion assembly 20 b therefore, for sake of efficiency, certainfeatures will be disclosed only with regard to propulsion assembly 20 a.One having ordinary skill in the art, however, will fully appreciate anunderstanding of propulsion assembly 20 b based upon the disclosureherein of propulsion assembly 20 a. Propulsion system 20 a includes anengine 30 that is fixed relative to wing 18. An engine output shaft 32transfers power from engine 30 to a spiral bevel gearbox 34 thatincludes spiral bevel gears to change torque direction by 90 degreesfrom engine 30 to a fixed gearbox 36. Fixed gearbox 36 includes aplurality of gears, such as helical gears, in a gear train that arecoupled to an interconnect drive shaft 38 and a common shaft depicted asquill shaft 40. Torque is transferred to an input gear 42 in spindlegearbox 44 of proprotor gearbox 46 through quill shaft 40.

Interconnect drive shaft 38 provides a torque path that enables a singleengine to provide torque to both proprotors assemblies 26 a, 26 b in theevent of a failure of the other engine. In the illustrated embodiment,interconnect drive shaft 38 has a rotational axis 48 that is verticallylower and horizontally aft of a longitudinal axis of the spindle gearbox44 referred to herein as a conversion axis 50. Conversion axis 50 isparallel to a lengthwise axis 52 of wing 18. Referring in particular toFIG. 8, interconnect drive shaft 38 includes a plurality of segmentsthat share rotational axis 48. Locating interconnect drive shaft 38 aftof wing spar 54, which is a structural member of the airframe oftiltrotor aircraft 10, provides for optimal integration with fixedgearbox 36 without interfering with the primary torque transfer of quillshaft 40 between fixed gearbox 36 and spindle gearbox 44. Conversionaxis 50 of spindle gearbox 44 is parallel to rotational axis 48 ofinterconnect drive shaft 38 but located forward and above rotationalaxis 48.

As best seen in FIG. 4, proprotor assembly 26 a of propulsion system 20a includes a plurality of proprotor blades 56 coupled to a yoke 58 thatis coupled to a mast 60. Mast 60 is coupled to proprotor gearbox 46. Thecollective and/or cyclic pitch of proprotor blades 56 may be controlledresponsive to pilot input via actuators 62, swashplate 64 and pitchlinks 66.

Referring in particular to FIG. 5, proprotor gearbox 46 is configured totransfer power and reduce speed to mast 60. Proprotor gearbox 46includes a top case portion 70 and spindle gearbox 44. Speed reductionis accomplished by a low speed planetary gear assembly 72 and a highspeed planetary gear assembly 74. A spiral bevel gear assembly includesspiral bevel input gear 42 and a spiral bevel output gear 76. The spiralbevel gear assembly changes power direction from along longitudinal axis50 of spiral bevel input gear 42 to a centerline axis 78 of spiral beveloutput gear 76. An accessory drive 80 can be coupled to spiral beveloutput gear 76. It should be appreciated that proprotor gearbox 46 caninclude additional or different components including bearing systems,lubrication systems and other gearbox related systems that may bebeneficial for operation.

During operation, a conversion actuator 80, as best seen in FIG. 4, canbe actuated so as to selectively rotate proprotor gearbox 46 and thuspylon assembly 24 a about conversion axis 50, which in turn selectivelypositions proprotor assembly 26 a between helicopter mode, as best seenin FIG. 2, and airplane mode, as best seen in FIGS. 1 and 3. Theoperational loads, such as thrust loads, are transmitted through mast 60and into spindle gearbox 44 of proprotor gearbox 46 and thus thestructural support of spindle gearbox 44 is critical. In the illustratedembodiment, spindle gearbox 44 is rotatably coupled to the airframe oftiltrotor aircraft 10 by mounting spindle gearbox 44 to an inboardpedestal depicted as inboard pillow block 82 having an inboard bearingassembly 86 and an outboard pedestal depicted as outboard pillow block84 with an outboard bearing assembly 88. Thus, spindle gearbox 44 isstructurally supported and is operable to be rotated about conversionaxis 50 by conversion actuator 80.

Inboard pillow block 82 is structurally coupled to an inboard tip rib90. Similarly, outboard pillow block 84 is structurally coupled to anoutboard tip rib 92. Inboard tip rib 90 and outboard tip rib 92 arestructural members of the airframe of tiltrotor aircraft 10. In theillustrated embodiment, the inboard pedestal includes an inboardintermediate support 94 that is utilized as a structural element betweeninboard pillow block 82 and inboard tip rib 90. Likewise, the outboardpedestal includes an outboard intermediate support 96 that is utilizedas a structural element between outboard pillow block 84 and outboardtip rib 92. It should be appreciated that the exact structuralconfiguration is implementation specific, and that structural componentscan be combined and/or separated to meet implementation specificrequirements. For example, in certain implementations, airframestructures such as tip ribs 90, 92 may extend above wing 18 and form aportion the inboard and outboard pedestals.

Pylon assembly 24 a including proprotor gearbox 46 and spindle gearbox44 is located above a surface of an upper wing skin 98 such thatconversion axis 50 is at a distance D1 above upper wing skin 98, as bestseen in FIG. 11. In addition, pylon assembly 24 a is generally centeredbetween inboard tip rib 90 and outboard tip rib 92. One advantage oflocating pylon assembly 24 a above the surface of upper wing skin 98 isthat the fore/aft location of pylon assembly 24 a can be easily tailoredto align the aircraft center of gravity (CG) with conversion axis 50while pylon assembly 24 a is in helicopter mode, while also aligning theaircraft center of gravity (CG) with the wing aerodynamic center of liftwhile pylon assembly 24 a is in airplane mode. It is noted that theaircraft center of gravity (CG) shifts as pylon assembly 24 a rotatesbetween helicopter mode and airplane mode. As such, locating pylonassembly 24 a above the wing allows the exact fore/aft location to beoptimized, while also structurally attaching pylon assembly 24 a to aportion of the airframe in the form of a torque box defined by forwardwing spar 100, aft wing spar 54, inboard tip rib 90 and outboard tip rib92.

The location of the spindle gearbox 44 provides an efficient structuralsupport for enduring operational loads by being mounted within thestructural torque box. For example, when aircraft 10 is in helicoptermode, torque about mast axis 78 is reacted by the torque box. It shouldbe noted that location of spindle gearbox 44 positions mast axis 78,while in helicopter mode, inboard of outboard tip rib 92, outboard ofinboard tip rib 90, forward of aft spar 54 and aft of forward spar 100,which allows the axis of the torque to be inside of the torque boxstructure, rather than cantilevered outside of the torque box structure.In contrast, a spindle gearbox location outside (such as outboard,forward or aft) would cause a moment that would increase operationalloading, thus requiring heavier and less efficient structural support.

Fixed gearbox 36 extends generally normal to conversion axis 50 and iscoupled to the airframe by a support assembly preferably having multiplejoints. In the illustrated embodiment, the support assembly includes afixed joint depicted as a bolted connection to a housing 102 that issupported by the airframe of tiltrotor aircraft 10 via outboard pillowblock 84 and outboard intermediate support 96. As illustrated, housing102 is a conical structure with one or more flanges configured tosupport bolted connections with fixed gearbox 36 and with outboardpillow block 84. The support assembly also includes one or more joints104 that provide support between fixed gearbox 36 and the airframe oftiltrotor aircraft 10, only one of which being visible in FIG. 9. It isnoted that joint 102 is the primary support structure between fixedgearbox 36 and the airframe. This is significant because the supportassembly is configured to maintain collinear alignment between fixedgearbox 36 and spindle gearbox 44. If the primary attachment structurewas not common with the attachment structure of spindle gearbox 44, thenoperation loading, such as load deflection and/or thermal growth, woulddramatically increase the potential for misalignment therebetween.Joints 104 may be stiff in certain directions but soft in otherdirections such as stiff in the inboard/outboard and verticaldirections, but soft in the fore/aft direction and/or stiff in theinboard/outboard and fore/aft directions, but soft in the verticaldirection.

Power is transferred from an output gear 106 of fixed gearbox 36 toinput gear 42 of spindle gearbox 44 through quill shaft 40. Quill shaft40 is a floating shaft configured to accept certain misalignment due tomanufacturing tolerances and operational effects between fixed gearbox36 and rotating spindle gearbox 44. Quill shaft 40 is configured to beassembled and disassembled independently from fixed gearbox 36 androtating spindle gearbox 44. As such, quill shaft 40 can be removedwithout removing either of fixed gearbox 36 or rotating spindle gearbox44.

Referring also to FIGS. 12-14, quill shaft 40 has a first splinedportion 110 and a second splined portion 112. In the illustratedembodiment, first splined portion 110 has a smaller diameter than secondsplined portion 112, thus first splined portion 110 is located inboardand second splined portion 112 is located outboard so that quill shaft40 can be removed to the outboard direction for inspection/maintenancethereof. Quill shaft 40 includes one or more inboard lubrication ports114 and outboard lubrication ports 116. Quill shaft 40 also includes afirst set of o-ring glands 118 and a second set of o-ring glands 120.

During operation, second splined portion 112 is in torque engagementwith output gear 106 of fixed gearbox 36 while first splined portion 110is in torque engagement with input gear 42 of spindle gearbox 44. In theillustrated embodiment, first splined portion 110 and second splinedportion 112 are crowned to promote teeth engagement in the event ofcollinear misalignment between spindle gearbox 44 and fixed gearbox 36.Lubrication oil is circulated to the mating surfaces of first splinedportion 110 through inboard lubrication ports 114, the seals associatedwith the first set of o-ring glands 118 forcing the lubrication fluid toflow to the first splined portion 110 instead of flowing toward thecenter of quill shaft 40. Similarly, lubrication oil is circulated tothe mating surfaces of the second splined portion 112 through outboardlubrication ports 116, the seals associated with the second set ofo-ring glands 120 forcing the lubrication fluid to flow to secondsplined portion 112 instead of flowing toward the center of quill shaft40.

One unique aspect of the configuration of quill shaft 40 in conjunctionwith spindle gearbox 44 and fixed gearbox 36 is that quill shaft 40 canbe removed without removing either of the spindle gearbox 44 or fixedgearbox 36. An access cover 122 can be removed thereby accessing thesecond splined portion 112 of quill shaft 40. An interior portion 124includes a feature, such as threads, for which a removal tool 126 canattach thereto. In one embodiment, interior portion 124 has femalethreads, while removal tool 126 has male threads that mate thereto. Uponattachment of removal tool 126, quill shaft 40 can be removed by pullingin an outboard direction along the centerline axis of quill shaft 40.Quill shaft 40 is critical for the operation of aircraft 10, as such,safety and efficiency of operation is improved by increasing the easefor which quill shaft 40 can be inspected.

Referring next to FIGS. 15A-15B of the drawings, therein is depicted amounting implementation for a pylon assembly above a wing of a tiltrotoraircraft. For the present discussion, only proprotor gearbox 46 andspindle gearbox 44 of pylon assembly 24 a have been shown. Pylonassembly 24 a is rotatably coupled between outboard pedestal 150 andinboard pedestal 152. In the illustrated embodiment, outboard pedestal150 includes a full pillow block housing 154 that provides a fulldiameter case to receive a bearing assembly 156 therein. Bearingassembly 156 may be a journal bearing assembly, a spherical bearingassembly or other suitable bearing assembly type. Bearing assembly 156is coupled to full pillow block housing 154 using bolts or othersuitable fasteners and provides a low friction environment for rotationof spindle gearbox 44 about conversion axis 50. In the illustratedembodiment, outboard pedestal 150 includes an upper portion of outboardtip rib 158 that extends above wing 18. Full pillow block housing 154 iscoupled to outboard tip rib 158 using suitable fasteners depicted as aplurality of bolts 160. As best seen in FIG. 15B, outboard tip rib 158preferably includes a sheer boss 162 that mates with a close fittingcavity in the lower surface of full pillow block housing 154.

In the illustrated embodiment, inboard pedestal 152 includes a fullpillow block housing 164 that provides a full diameter case to receive abearing assembly 166 therein. Bearing assembly 166 may be a journalbearing assembly, a spherical bearing assembly or other suitable bearingassembly type. Bearing assembly 166 is coupled to full pillow blockhousing 164 using bolts or other suitable fasteners and provides a lowfriction environment for rotation of spindle gearbox 44 about conversionaxis 50. In the illustrated embodiment, inboard pedestal 152 includes anupper portion of inboard tip rib 168 that extends above wing 18. Fullpillow block housing 164 is coupled to inboard tip rib 168 usingsuitable fasteners depicted as a plurality of bolts 170. As best seen inFIG. 15B, inboard tip rib 168 preferably includes a sheer boss 172 thatmates with a close fitting cavity in the lower surface of full pillowblock housing 164. Even though the lower sections of outboard pedestal150 and inboard pedestal 152 have been described as being integral withoutboard tip rib 158 and inboard tip rib 168, those having ordinaryskill in the art will recognize that the lower sections of outboardpedestal 150 and inboard pedestal 152 could alternatively include one ormore intermediate supports, in which case, item 158 would represent anoutboard intermediate support such as outboard intermediate support 96discussed above and item 168 would represent an inboard intermediatesupport such as inboard intermediate support 94 discussed above.

It is desirable to be able to remove pylon assembly 24 a verticallyrelative to wing 18 for inspection, maintenance or other protocols. Asbest seen in FIG. 15B, full pillow block housing 154 can be separatedfrom outboard tip rib 158 by removing bolts 160 and full pillow blockhousing 164 can be separated from inboard tip rib 168 by removing bolts170. Pylon assembly 24 a together with full pillow block housings 154,164 may then be vertically lifted off tip ribs 158, 168. Full pillowblock housing 154 together with bearing assembly 156 and full pillowblock housing 164 together with bearing assembly 166 may then belaterally removed from spindle gearbox 44. This procedure may bereversed to install pylon assembly 24 a on the tiltrotor aircraft. Fullpillow block housing 154 together with bearing assembly 156 and fullpillow block housing 164 together with bearing assembly 166 arelaterally mounted to spindle gearbox 44. Thereafter, pylon assembly 24 atogether with full pillow block housings 154, 164 are lowered onto tipribs 158, 168 such that sheer boss 162 mates with the close fittingcavity in the lower surface of full pillow block housing 154 and sheerboss 172 mates with the close fitting cavity in the lower surface offull pillow block housing 164. This arrangement aides in establishingproper alignment between spindle gearbox 44 and other criticalcomponents of the tiltrotor aircraft, such as fixed gearbox 36. Fullpillow block housing 154 can now be coupled to outboard tip rib 158 withbolts 160 and full pillow block housing 164 can now be coupled toinboard tip rib 168 with bolts 170.

In operation, outboard pedestal 150 and inboard pedestal 152 mustsupport fore/aft loads generated by the proprotor assembly when thetiltrotor aircraft is cruising in airplane mode. In the illustratedembodiment, the shear forces between outboard tip rib 158 and fullpillow block housing 154 react on sheer boss 162 which not only acts tomaintain collinear alignment between output gear 106 of fixed gearbox 36and input gear 42 of spindle gearbox 44 but also prevents shear forcesfrom acting on bolts 160. Likewise, the shear forces between inboard tiprib 168 and full pillow block housing 164 react on sheer boss 172 whichprevents shear forces from acting on bolts 170. Outboard pedestal 150and inboard pedestal 152 must also support vertical loads during allflight operations including peak vertical loads that are generated bythe proprotor assembly when the tiltrotor aircraft is in helicoptermode. In the illustrated embodiment, bolts 160 react to support tensionforces between outboard tip rib 158 and full pillow block housing 154while bolts 170 react to support tension forces between inboard tip rib168 and full pillow block housing 164. In addition, bolts 160 must reactto support certain bending moments between outboard tip rib 158 and fullpillow block housing 154 while bolts 170 must react to support certainbending moments between inboard tip rib 168 and full pillow blockhousing 164. These bending moments may be generated due to lateralmovements or vibrations caused by operating modes of the proprotorassembly.

Use of full pillow block housings in pedestals 150, 152 enables verticalremoval and installation of pylon assembly 24 a. In addition, the fullpillow block housings enable final installation of the bearingassemblies within the full pillow block housings prior to theinstallation of pylon assembly 24 a within the full pillow blockhousings. The maximum stiffness of outboard pedestal 150 and inboardpedestal 152 is limited, however, due to the split lines in outboardpedestal 150 and inboard pedestal 152 between the full pillow blockhousings and the tip ribs. If greater stiffness in the coupling betweenpylon assembly 24 a and the airframe is desired, one or both of theoutboard and inboard pedestals may be modified as discussed below.

For example, referring next to FIGS. 16A-16B of the drawings, pylonassembly 24 a is rotatably coupled between outboard pedestal 180 andinboard pedestal 182. In the illustrated embodiment, outboard pedestal180 is depicted as a split pillow block housing including a pillow blockcap 184 and a pillow block base 186 that together provide a splitdiameter case to receive a bearing assembly 188 therein. Bearingassembly 188 may be a journal bearing assembly, a spherical bearingassembly or other suitable bearing assembly type. Bearing assembly 188is coupled to pillow block cap 184 and pillow block base 186 using boltsor other suitable fasteners and provides a low friction environment forrotation of spindle gearbox 44 about conversion axis 50. In theillustrated embodiment, pillow block base 186 is integral with and formsan upper portion of an outboard tip rib extending above wing 18.Alternatively, pillow block base 186 may be coupled to an outboard tiprib disposed within or partially within wing 18. Pillow block cap 184and pillow block base 186 are coupled together using suitable fastenersdepicted as a plurality of bolts 190.

In the illustrated embodiment, inboard pedestal 182 includes a fullpillow block housing 194 that provides a full diameter case to receive abearing assembly 196 therein. Bearing assembly 196 may be a journalbearing assembly, a spherical bearing assembly or other suitable bearingassembly type. Bearing assembly 196 is coupled to full pillow blockhousing 194 using bolts or other suitable fasteners and provides a lowfriction environment for rotation of spindle gearbox 44 about conversionaxis 50. In the illustrated embodiment, inboard pedestal 182 includes anupper portion of inboard tip rib 198 that extends above wing 18. Fullpillow block housing 194 is coupled to inboard tip rib 198 usingsuitable fasteners depicted as a plurality of bolts 200. As best seen inFIG. 16B, inboard tip rib 198 preferably includes a sheer boss 202 thatmates with a close fitting cavity in the lower surface of full pillowblock housing 194.

It is desirable to be able to remove pylon assembly 24 a verticallyrelative to wing 18 for inspection, maintenance or other protocols. Asbest seen in FIG. 16B, pillow block cap 184 can be separated from pillowblock base 186 by removing bolts 190 and by removing the bolts thatcouple bearing assembly 188 to pillow block base 186. Full pillow blockhousing 194 can be separated from inboard tip rib 198 by removing bolts200. Pylon assembly 24 a together with pillow block cap 184, bearingassembly 188 and full pillow block housings 194 may then be verticallylifted off tip ribs 186, 198. Pillow block cap 184 together with bearingassembly 188 and full pillow block housing 194 together with bearingassembly 196 may then be laterally removed from spindle gearbox 44. Thisprocedure may be reversed to install pylon assembly 24 a on thetiltrotor aircraft. Pillow block cap 184 together with bearing assembly188 and full pillow block housing 194 together with bearing assembly 196are laterally mounted to spindle gearbox 44. Thereafter, pylon assembly24 a together with pillow block cap 184, bearing assembly 188 and fullpillow block housings 194 are lowered onto tip ribs 186, 198 such thatsheer boss 202 mates with the close fitting cavity in the lower surfaceof full pillow block housing 194. Pillow block cap 184 can now becoupled to pillow block base 186 with bolts 190 and bearing assembly 188can also be coupled to pillow block base 186. In addition, full pillowblock housing 194 can now be coupled to inboard tip rib 198 with bolts200.

In operation, outboard pedestal 180 and inboard pedestal 182 mustsupport fore/aft loads and vertical loads generated by the proprotorassembly when the tiltrotor aircraft is cruising in airplane mode and/oroperating in helicopter mode. In the illustrated embodiment, the loadspectrum on bolts 190 includes sheer forces and tension forces butminimal bending moments as the split line between pillow block cap 184and pillow block base 186 is coincident with the centerline of spindlegearbox 44. The load spectrum on bolts 200 includes tension forces andbending moments but minimal shear forces which react instead on sheerboss 202. Use of a split pillow block housing in outboard pedestal 180and a full pillow block housings in inboard pedestal 182 enablesvertical removal and installation of pylon assembly 24 a. In addition,the split pillow block housing including a tip rib as the pillow blockbase enables stiffness tailoring of outboard pedestal 180 to achievedesired dynamic modes and to maintain the output gear of fixed gearbox36 in substantial collinear alignment with the input gear of spindlegearbox 44.

Referring next to FIGS. 17A-17B of the drawings, pylon assembly 24 a isrotatably coupled between outboard pedestal 210 and inboard pedestal212. In the illustrated embodiment, outboard pedestal 210 is depicted asa split pillow block housing including a pillow block cap 214 and apillow block base 216 that together provide a split diameter case toreceive a bearing assembly 218 therein. Bearing assembly 218 may be ajournal bearing assembly, a spherical bearing assembly or other suitablebearing assembly type. Bearing assembly 218 is coupled to pillow blockcap 214 and pillow block base 216 using bolts or other suitablefasteners and provides a low friction environment for rotation ofspindle gearbox 44 about conversion axis 50. In the illustratedembodiment, pillow block base 216 is integral with and forms an upperportion of an outboard tip rib extending above wing 18. Pillow block cap214 and pillow block base 216 are coupled together using suitablefasteners depicted as a plurality of bolts 220.

In the illustrated embodiment, inboard pedestal 212 is depicted as asplit pillow block housing including a pillow block cap 224 and a pillowblock base 226 that together provide a split diameter case to receive abearing assembly 228 therein. Bearing assembly 228 may be a journalbearing assembly, a spherical bearing assembly or other suitable bearingassembly type. Bearing assembly 228 is coupled to pillow block cap 224and pillow block base 226 using bolts or other suitable fasteners andprovides a low friction environment for rotation of spindle gearbox 44about conversion axis 50. In the illustrated embodiment, pillow blockbase 226 is integral with and forms an upper portion of an inboard tiprib extending above wing 18. Pillow block cap 224 and pillow block base226 are coupled together using suitable fasteners depicted as aplurality of bolts 230.

It is desirable to be able to remove pylon assembly 24 a verticallyrelative to wing 18 for inspection, maintenance or other protocols. Asbest seen in FIG. 17B, pillow block cap 214 can be separated from pillowblock base 216 by removing bolts 220 and by removing the bolts thatcouple bearing assembly 218 pillow block base 216. Likewise, pillowblock cap 224 can be separated from pillow block base 226 by removingbolts 230 and by removing the bolts that couple bearing assembly 228pillow block base 226. Pylon assembly 24 a together with pillow blockcap 214, bearing assembly 218, pillow block cap 224 and bearing assembly228 may then be vertically lifted off tip ribs 216, 226. Pillow blockcap 214 together with bearing assembly 218 and pillow block cap 224together with bearing assembly 228 may then be laterally removed fromspindle gearbox 44. This procedure may be reversed to install pylonassembly 24 a on the tiltrotor aircraft. Pillow block cap 214 togetherwith bearing assembly 218 and pillow block cap 224 together with bearingassembly 228 are laterally mounted to spindle gearbox 44. Thereafter,pylon assembly 24 a together with pillow block cap 214, bearing assembly218, pillow block cap 224 and bearing assembly 228 are lowered onto tipribs 216, 226. Pillow block cap 214 can now be coupled to pillow blockbase 216 with bolts 220 and bearing assembly 218 can also be coupled topillow block base 216. In addition, pillow block cap 224 can now becoupled to pillow block base 226 with bolts 230 and bearing assembly 228can also be coupled to pillow block base 226.

In operation, outboard pedestal 210 and inboard pedestal 212 mustsupport fore/aft loads and vertical loads generated by the proprotorassembly when the tiltrotor aircraft is cruising in airplane mode and/oroperating in helicopter mode. In the illustrated embodiment, the loadspectrum on bolts 220, 230 includes sheer forces and tension forces butminimal bending moments as the split lines between the pillow block capsand the pillow block bases are coincident with the centerline of spindlegearbox 44. Use of split pillow block housings in pedestals 210, 212enables vertical removal and installation of pylon assembly 24 a. Inaddition, the split pillow block housings including tip ribs as thepillow block bases enable stiffness tailoring of outboard pedestal 210and inboard pedestal 212 to achieve desired dynamic modes and tomaintain the output gear of fixed gearbox 36 in substantial collinearalignment with the input gear of spindle gearbox 44.

Referring next to FIGS. 18A-18B of the drawings, pylon assembly 24 a isrotatably coupled between outboard pedestal 240 and inboard pedestal242. In the illustrated embodiment, outboard pedestal 240 including abearing cartridge 244 and a tip rib 246. Bearing cartridge 244 providesa full diameter case to receive a bearing assembly 248 therein. Bearingassembly 248 may be a journal bearing assembly, a spherical bearingassembly or other suitable bearing assembly type. Bearing assembly 248is coupled to bearing cartridge 244 using bolts or other suitablefasteners and provides a low friction environment for rotation ofspindle gearbox 44 about conversion axis 50. Bearing cartridge 244 andtip rib 246 are coupled together using suitable fasteners depicted as aplurality of bolts 250. As best seen in FIG. 18B, tip rib 246 defines aslot 252 that is designed to closely receive bearing cartridge 244therein. In one example, tip rib 246 and bearing cartridge 244 areprecision machined aluminum components having tight tolerances such thatthe close fitting relationship between slot 252 and bearing cartridge244 is achieved.

In the illustrated embodiment, inboard pedestal 242 including a bearingcartridge 254 and a tip rib 256. Bearing cartridge 254 provides a fulldiameter case to receive a bearing assembly 258 therein. Bearingassembly 258 may be a journal bearing assembly, a spherical bearingassembly or other suitable bearing assembly type. Bearing assembly 258is coupled to bearing cartridge 254 using bolts or other suitablefasteners and provides a low friction environment for rotation ofspindle gearbox 44 about conversion axis 50. Bearing cartridge 254 andtip rib 256 are coupled together using suitable fasteners depicted as aplurality of bolts 260. As best seen in FIG. 18B, tip rib 256 defines aslot 262 that is designed to closely receive bearing cartridge 254therein. In one example, tip rib 256 and bearing cartridge 254 areprecision machined aluminum components having tight tolerances such thatthe close fitting relationship between slot 262 and bearing cartridge254 is achieved.

It is desirable to be able to remove pylon assembly 24 a verticallyrelative to wing 18 for inspection, maintenance or other protocols. Asbest seen in FIG. 18B, bearing cartridge 244 can be separated from tiprib 246 by removing bolts 250 and bearing cartridge 254 can be separatedfrom tip rib 256 by removing bolts 260. Pylon assembly 24 a togetherwith bearing cartridges 244, 254 may then be vertically lifted out oftip ribs 246, 256. Bearing cartridge 244 along with bearing assembly 248and bearing cartridge 254 along with bearing assembly 258 may then belaterally removed from spindle gearbox 44. This procedure may bereversed to install pylon assembly 24 a on the tiltrotor aircraft.Bearing cartridge 244 along with bearing assembly 248 and bearingcartridge 254 along with bearing assembly 258 are laterally mounted tospindle gearbox 44. Thereafter, pylon assembly 24 a together withbearing cartridges 244, 254 are lowered into slots 252, 262 of tip ribs246, 256. Bearing cartridge 244 can now be coupled to tip rib 246 withbolts 250 and bearing cartridge 254 can now be coupled to tip rib 256with bolts 260.

In operation, outboard pedestal 240 and inboard pedestal 242 mustsupport fore/aft loads and vertical loads generated by the proprotorassembly when the tiltrotor aircraft is cruising in airplane mode and/oroperating in helicopter mode. The primary loads on bolts 250, 260 aresheer forces in the vertical direction. Use of bearing cartridgesreceived in tip rib slots to form outboard pedestal 240 and inboardpedestal 242 enables vertical removal and installation of pylon assembly24 a. In addition, the bearing cartridges provides a single concentricdiameter for mounting the bearing assemblies therein which also enablesfinal installation of the bearing assemblies prior to the installationof pylon assembly 24 a therewith. In addition, the close fittingrelationship between the bearing cartridges and the tip ribs provides astiff coupling therebetween. Further, the use of the tip ribs to receivethe bearing cartridges enables stiffness tailoring of outboard pedestal240 and inboard pedestal 242 to achieve desired dynamic modes and tomaintain the output gear of fixed gearbox 36 in substantial collinearalignment with the input gear of spindle gearbox 44.

Referring next to FIG. 19 of the drawings, therein is depicted amounting implementation for a pylon assembly above a wing of a tiltrotoraircraft. For the present discussion, only spindle gearbox 44 of pylonassembly 24 a has been shown. Pylon assembly 24 a is rotatably coupledbetween outboard pedestal 270 and inboard pedestal 272. Outboardpedestal 270 and inboard pedestal 272 may be any type of above-wingstructure to which the pylon assembly is mounted including, for example,pedestals having full pillow block housings, split pillow block housingsand/or bearing cartridges, as discussed herein. In the illustratedembodiment, outboard pedestal 270 includes a bearing assembly depictedas a journal bearing assembly 274 and inboard pedestal 272 includes abearing assembly depicted as a journal bearing assembly 276. Journalbearing assembly 274 is coupled to outboard pedestal 270 and preferablyhas low friction contact surface 278. Likewise, journal bearing assembly276 is coupled to inboard pedestal 272 and preferably has low frictioncontact surface 280. Journal bearing assemblies 274, 276 providing astiff coupling between pylon assembly 24 a and pedestals 270, 272 tocontrol dynamic modes between pylon assembly 24 a and the airframe andto maintain the output gear of fixed gearbox 36 in substantial collinearalignment with the input gear of spindle gearbox 44.

Journal bearing assembly 274 is a fixed bearing that substantiallyprevents lateral movement of pylon assembly 24 a relative to outboardpedestal 270. In the illustrated embodiment, this is achieved using aspacer 282, a lock washer 284 and a spanner nut 286 that threadablycouples with a sleeve 288 of spindle gearbox 44 to the inboard side ofoutboard pedestal 270. To the outboard side, a thrust washer/clamp ring290 is coupled to spindle gearbox 44 by bolting or other suitableconnection. Preferably, spacer 282 and sleeve 288 have low frictioncontact surfaces with journal bearing assembly 274. In addition, thrustwasher/clamp ring 290 preferably has a low friction contact surface withoutboard pedestal 270. Journal bearing assembly 276 is a floatingbearing that allows lateral movement of pylon assembly 24 a relative toinboard pedestal 272. Preferably, pylon assembly 24 a includes sleeve292 that has a low friction contact surface with journal bearingassembly 276. In operation, when spindle gearbox 44 is rotated tooperate tiltrotor aircraft 10 between helicopter and airplane modes,spacer 282, lock washer 284, spanner nut 286 and thrust washer/clampring 290 as well as sleeve 288 and sleeve 292 rotate with spindlegearbox 44 relative to journal bearing assemblies 274, 276 and thuspedestals 270, 272.

Referring next to FIG. 20 of the drawings, therein is depicted amounting implementation for a pylon assembly above a wing of a tiltrotoraircraft. Pylon assembly 24 a is rotatably coupled between outboardpedestal 300 and inboard pedestal 302, which may be any type ofabove-wing structure to which the pylon assembly is mounted includingfor example, pedestals having full pillow block housings, split pillowblock housings and/or bearing cartridges as discussed herein. In theillustrated embodiment, outboard pedestal 300 includes a bearingassembly depicted as a spherical bearing assembly 304 and inboardpedestal 302 includes a bearing assembly depicted as a spherical bearingassembly 306. Spherical bearing assembly 304 includes a spherical race308 that is coupled to outboard pedestal 300 and a monoball 310 that isrotatable relative to spherical race 308. Spherical race 308 andmonoball 310 preferably have low friction contact surfaces to provide alow friction environment for relative rotation. Likewise, sphericalbearing assembly 306 includes a spherical race 312 that is coupled toinboard pedestal 302 and a monoball 314 that is rotatable relative tospherical race 312. Spherical race 312 and monoball 314 preferably havelow friction contact surfaces to provide a low friction environment forrelative rotation. Spherical bearing assemblies 304, 306 provide aself-aligning coupling between pylon assembly 24 a and pedestals 300,302 that reduces the alignment sensitivity of pedestals 300, 302 andimproves the installation repeatability of pylon assembly 24 a inpedestals 300, 302. Spherical bearing assemblies 304, 306 are able toestablish an axis of rotation in an environment including certainmisalignment of pedestals 300, 302 and between pylon assembly 24 a andpedestals 300, 302. In addition, as spherical bearing assemblies 304,306 do not react to moments within the joint, spherical bearingassemblies 304, 306 have improved wear.

Spherical bearing assembly 304 is a fixed bearing that substantiallyprevents lateral movement of pylon assembly 24 a relative to outboardpedestal 300. In the illustrated embodiment, this is achieved using alock washer 316 and a spanner nut 318 that threadably couples with asleeve 320 of spindle gearbox 44 to the inboard side of outboardpedestal 300 and a thrust washer/clamp ring 324 to the outboard side ofoutboard pedestal 300. An optional spacer 326 may be positioned betweenspherical bearing assembly 304 and lock washer 316. Spherical bearingassembly 306 is a floating bearing that allows lateral movement of pylonassembly 24 a relative to inboard pedestal 302. Preferably, pylonassembly 24 a includes sleeve 328 that is positioned within sphericalbearing assembly 306. In operation, when spindle gearbox 44 is rotatedto operate tiltrotor aircraft 10 between helicopter and airplane modes,spacer 326, lock washer 316, spanner nut 318 and thrust washer/clampring 324 as well as sleeve 320, monoball 310, sleeve 328 and monoball314 rotate with spindle gearbox 44 relative to spherical races 308, 312and thus pedestals 300, 302.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. A propulsion system for a tiltrotor aircrafthaving a helicopter mode and an airplane mode, the tiltrotor aircrafthaving an airframe including a fuselage and a wing, the propulsionsystem comprising: an engine supported by the airframe proximate anoutboard end of the wing; a fixed gearbox operably coupled to the engineand having an output gear; inboard and outboard pedestals supported bythe airframe and positioned above the wing; a pylon assembly rotatablycoupled between the inboard and outboard pedestals, the pylon assemblyincluding a spindle gearbox having an input gear, a mast operablycoupled to the input gear and a proprotor assembly operable to rotatewith the mast, the spindle gearbox rotatable about a conversion axis toselectively operate the tiltrotor aircraft between the helicopter modeand the airplane mode; and a common shaft configured to transfer torquefrom the output gear of the fixed gearbox to the input gear of thespindle gearbox, the common shaft rotatable about the conversion axis;wherein, each of the inboard and outboard pedestals further comprises aspherical bearing providing a self-aligning coupling between the pylonassembly and the inboard and outboard pedestals, thereby reducingalignment sensitivity between the inboard and outboard pedestals.
 2. Thepropulsion system as recited in claim 1 wherein the coupling between thepylon assembly and the outboard pedestal further comprises a fixedbearing coupling to substantially prevent lateral movement of the pylonassembly relative to the outboard pedestal.
 3. The propulsion system asrecited in claim 1 wherein the coupling between the pylon assembly andthe inboard pedestal further comprises a floating bearing coupling toallow lateral movement of the pylon assembly relative to the inboardpedestal.
 4. The propulsion system as recited in claim 1 wherein thespherical bearings enable the spindle gearbox to rotate about theconversion axis in an environment including misalignment of the inboardand outboard pedestals.
 5. The propulsion system as recited in claim 1wherein each of the spherical bearings further comprises a monoballbearing and a spherical race.
 6. The propulsion system as recited inclaim 1 wherein the pylon assembly further comprises an inboard sleevepositioned within the spherical bearing of the inboard pedestal and anoutboard sleeve positioned within the spherical bearing of the outboardpedestal.
 7. The propulsion system as recited in claim 1 wherein each ofthe inboard and outboard pedestals further comprises a full pillow blockhousing.
 8. The propulsion system as recited in claim 1 wherein each ofthe inboard and outboard pedestals further comprises a split pillowblock housing.
 9. The propulsion system as recited in claim 1 whereinthe inboard pedestal further comprises a full pillow block housing andwherein the outboard pedestal further comprises a split pillow blockhousing.
 10. The propulsion system as recited in claim 1 wherein each ofthe inboard and outboard pedestals further comprises a tip rib extendingabove the wing and defining a slot and a bearing cartridge including abearing assembly received within the slot.
 11. The propulsion system asrecited in claim 1 wherein the inboard and outboard pedestals supportfore/aft loads generated by the proprotor assembly when the tiltrotoraircraft is in the airplane mode.
 12. The propulsion system as recitedin claim 1 wherein the inboard and outboard pedestals support verticalloads generated by the proprotor assembly when the tiltrotor aircraft isin the helicopter mode.
 13. The propulsion system as recited in claim 1wherein the fixed gearbox extends generally normal to the conversionaxis and wherein the fixed gearbox is coupled to the outboard pedestalto maintain the output gear of the fixed gearbox in substantialcollinear alignment with the input gear of the spindle gearbox.
 14. Atiltrotor aircraft having a helicopter mode and an airplane mode, thetiltrotor aircraft comprising: an airframe including a fuselage and awing; an engine supported by the airframe proximate an outboard end ofthe wing; a fixed gearbox operably coupled to the engine and having anoutput gear; inboard and outboard pedestals supported by the airframeand positioned above the wing; a pylon assembly rotatably coupledbetween the inboard and outboard pedestals, the pylon assembly includinga spindle gearbox having an input gear, a mast operably coupled to theinput gear and a proprotor assembly operable to rotate with the mast,the spindle gearbox rotatable about a conversion axis to selectivelyoperate the tiltrotor aircraft between the helicopter mode and theairplane mode; and a common shaft configured to transfer torque from theoutput gear of the fixed gearbox to the input gear of the spindlegearbox, the common shaft rotatable about the conversion axis; wherein,each of the inboard and outboard pedestals further comprises a sphericalbearing providing a self-aligning coupling between the pylon assemblyand the inboard and outboard pedestals, thereby reducing alignmentsensitivity between the inboard and outboard pedestals.
 15. Thetiltrotor aircraft as recited in claim 14 wherein the coupling betweenthe pylon assembly and the outboard pedestal further comprises a fixedbearing coupling to substantially prevent lateral movement of the pylonassembly relative to the outboard pedestal and wherein the couplingbetween the pylon assembly and the inboard pedestal further comprises afloating bearing coupling to allow lateral movement of the pylonassembly relative to the inboard pedestal.
 16. The tiltrotor aircraft asrecited in claim 14 wherein the spherical bearings enable the spindlegearbox to rotate about the conversion axis in an environment includingmisalignment of the inboard and outboard pedestals.
 17. The tiltrotoraircraft as recited in claim 14 wherein each of the spherical bearingsfurther comprises a monoball bearing and a spherical race.
 18. Thetiltrotor aircraft as recited in claim 14 wherein, the inboard pedestalis selected from the group consisting of a full pillow block housing anda split pillow block housing and wherein, the outboard pedestal isselected from the group consisting of a full pillow block housing and asplit pillow block housing.
 19. The tiltrotor aircraft as recited inclaim 14 wherein each of the inboard and outboard pedestals furthercomprises a tip rib extending above the wing and defining a slot and abearing cartridge including a bearing assembly received within the slot.20. The tiltrotor aircraft as recited in claim 14 wherein the fixedgearbox extends generally normal to the conversion axis and wherein thefixed gearbox is coupled to the outboard pedestal to maintain the outputgear of the fixed gearbox in substantial collinear alignment with theinput gear of the spindle gearbox.